Pressure applied to silver halide photographic emulsion coatings can produce both reversible and irreversible effects on the sensitometry of the photographic product. Various types of pressure effects on silver halide photographic systems have been known for long periods of time. In general, pressure sensitivity can be described as an effect which causes the photographic sensitometry of film products to change after the application of some kind of a mechanical stress to a coated photographic film. Sufficient pressure can cause irreversible distortion of the emulsion grains or cause the formation of physical defects that alter the sensitivity for latent image formation. The prior art, such as described in James, The Theory of the Photographic Process, 4th Ed., MacMillan (1977), describe various mechanisms in association with the various types of pressure sensitivities observed with photographic products, wherein the transmission of mechanical and thermal stress to silver halide crystals cause a change in sensitometry for the photographic products.
Pressure sensitivity may manifest itself in photographic products in the form of pressure desensitization or pressure fog, resulting in decreased or increased density marks, respectively, after development. Pressure fog, which is often called photoabrasion, is an increasingly large impediment to the manufacture and use of photographic recording materials. The problem is generally believed to arise from large local stresses applied to the recording materials when small particles of dirt on transport mechanism rollers are pressed against the materials in cameras or other exposing devises or possibly during processing operations.
Attempts to control this problem include use of gelatin overcoat layers. Such layers, however, even if relatively thick as disclosed in Japanese Kokais 01-267638(1989) and 01-291251(1989), do not offer adequate protection themselves. Dry gelatin is hard and can thus easily transmit applied stress to the silver halide crystals in a coated photographic system. Japanese Kokai 01-61748(1989) discloses the use of protective overcoat layers containing colloidal silica and an ester on photographic elements in order to improve pressure fog resistance, and discloses that synthetic polymer latexes may be present in the emulsion or other layers of the photographic elements. U.S. Pat. No. 4,464,462 discloses that the presense of an ultraviolet ray absorbing polymer latex in a photographic element does not have an adverse influence on fog, but there is no teaching that the occurance of pressure fog is decreased by its presense.
The prior art also describes the inclusion of polymer latexes into coated emulsion layers to decrease pressure desensitization in photographic products as disclosed in U.S. Pat. No.3,576,628, to distribute hydrophobic addenda in a hydrophilic colloid layer as disclosed in U.S. Pat. No. 4,247,627, and as plasticizers for gelatin as described, for example, in U.S. Pat. No. 4,245,036. While the inclusion of polymer latexes in emulsion layers may help reduce pressure desensitization problems, this approach has generally caused an increase in the pressure fog problem. The prior art also describes in U.S. Pat. Nos. 4,551,412 and 4,822,727 the use of polymer latexes having glass transition temperatures of both above 20.degree. C. and below 20.degree. C. in overcoat layers in order to decrease brittleness and reticulation while improving sticking resistance in photographic elements. Similarly, the prior art describes the use of organic solvent dispersions in photographic silver halide emulsion and overcoat layers as disclosed in U.S. Pat. Nos. 4,840,881, 4,499,179, and 4,399,213.
In general, pressure sensitivity problems increase with the physical size of the emulsion crystals. Its manifestation is most severe in the high aspect ratio highly deformable "Tabular Grain Emulsions," used in many current photographic products and extensively described in prior art. There is, therefore, a need to produce photographic elements that are less sensitive to mechanical stress in order to improve the quality of many of today's current photographic products. It would be desirable to reduce pressure fog in such photographic products without detrimentally affecting other photographic qualities, and while retaining good scratch resistance for photographic elements.